High-strength steel sheets have been suitably used as a material for automotive components or the like, for example, in order to reduce the weight of the components by reducing the thickness of the material. For example, frame components and crash-resistant components are required to be resistant to deformation at the time of impact in order to ensure the safety of vehicle occupants. That is, these components are required to have a high yield ratio. As a material for these components, high-strength steel sheets having excellent stretch flangeability are desirably used in order to consistently produce the components by press forming without the occurrence of cracking. There have been disclosed various steel sheets and techniques for producing the steel sheets in response to the above demand.
Patent Literature 1 discloses a high-strength steel sheet having excellent paint-baking hardenability, the steel sheet including Nb and Ti at a content of 0.01% or more in total and a main phase that is ferrite having a recrystallization ratio of 80% or more, and a method for producing the high-strength steel sheet.
Patent Literature 2 discloses a high-strength steel sheet having excellent collision resistance and excellent stretch flangeability, the high-strength steel sheet having a microstructure including 20% to 50% unrecrystallized ferrite, and a method for producing the high-strength steel sheet.
Patent Literature 3 discloses a hot-dip zinc coated high-strength steel sheet having excellent stretch flangeability (hole expandability), the high-strength steel sheet containing one or more elements selected from V, Ti, and Nb and including a main phase that is ferrite or bainite, wherein the amount of iron carbide precipitated at the grain boundaries is limited to be a specific amount and the maximum particle diameter of the iron carbide is controlled to be 1 μm or less, and a method for producing the hot-dip zinc coated high-strength steel sheet.